Numerous polymer-based implantable medical devices have been developed for implantation or insertion into the body. Examples of such medical devices include endoprosthetic joints, which typically include a metal or ceramic component articulating on or bearing against a polymeric article. On such endoprosthetic device is a knee prosthesis that includes a femoral knee prosthesis which articulates against a polymeric article. Such polymeric articles are typically made from, for example, polyethylene, ultra high molecular weight polyethylene (UHMWPE), polyaryletherketones, polyurethanes, or combinations and blends of such polymers.
In an effort to enhance certain characteristics of the polymeric article, such as hydrophilicity, wettability, lubricity, and wear resistance, just to name a few, the exterior or outer surfaces of the polymeric article may be modified to include or to have bonded thereto selected functional groups. Surface modification of a polymeric article is typically applied to an already formed polymeric article, which has been formed by, for example, compression molding, ram extrusion or deposition. After the polymeric article has been formed, an exterior surface is subjected to a modification process to introduce and bond functional or reactive groups to the polymer molecules on or near the exterior surface of the polymeric article. Exterior surface modification of a polymeric article may be accomplished by, for example, plasma treatment or wet or dry chemical treatments of the polymeric article's exterior surface.
In the orthopedic field, due to the articulation and load bearing functions of the polymeric article, over time, the exterior surface is subjected to increased wear. As a result the functional groups on the modified surface are worn off. When the original modified exterior surface is worn off, the inner bulk unmodified polymer material is exposed and, in effect, becomes the exterior surface, which may also be the articulating and load-bearing surface, of the polymeric article. Thus, when the modified exterior surface is worn off, the beneficial properties or characteristics provided by the functional groups and the modified surface generally are diminished if not lost.